Due to their high resolution, high quality images and independence from backlight sources, organic light emitting diodes (OLEDs) have attracted considerable attention in display and lighting applications.
An OLED electrically excites fluorescent or phosphorescent organic compounds to emit light. OLEDs may be either passive-matrix type or active-matrix type, depending upon the method utilized to drive the display pixels, which are arranged in a matrix.
While being commercially successful in small area applications, such as cell phone displays, large area organic devices have met technical difficulties. Since the anode and cathode layers are thin-films having limited electrical conductivity, they are not able to carry high current without substantial energy loss. This problem is further accentuated when one of the electrode layers also has to be optically transparent to allow light to pass. For OLED lamps this problems is even more relevant since they operate at relatively high currents.
In order to solve this problem, metal shunts can be introduced which compensate for the energy loss and serve as means for guiding and accelerating the propagation of the discharge.
Typically, metal shunts are separated from the cathode layer by means of an insulating resist layer.
Such an arrangement is disclosed in WO 2005/053053. This application relates to a light emitting device comprising a stack of substrate layers, an anode layer, a light emitting layer, a cathode layer, patterned metal shunts, and an etch-protective layer. Cathode isolation resist lines are arranged on the metal shunts which together define the pixel pattern of the device.
Although the invention disclosed in WO 2005/053053 solves a different problem, it illustrates a consequence of using resists to isolate metal shunts from a cathode layer. In order to function as electrically insulating layers, the resist layers arranged on top of the metal shunts need to extend over a larger surface than the top surface of the metal shunts. The result is that the effective pixel area, i.e. light transmitting surface, defined in between the shunts, is reduced
Thus, there is a need in the art to provide a light emitting device comprising metal shunt(s) which can be isolated from the cathode without reducing the effective pixel area.